Network Working Group H. Asaeda
Internet-Draft NICT
Intended status: Standards Track Q. Wu
Expires: August 30, 2014 R. Huang
Huawei
February 26, 2014
RTP Control Protocol (RTCP) Extended Report (XR) Blocks forSynchronization Delay and Offset Metrics Reportingdraft-ietf-xrblock-rtcp-xr-synchronization-09
Abstract
This document defines two RTP Control Protocol (RTCP) Extended Report
(XR) Blocks that allow the reporting of synchronization delay and
offset metrics for use in a range of RTP applications.
Status of this Memo
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This Internet-Draft will expire on August 30, 2014.
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Internet-Draft SDO Report Blocks February 20141. Introduction1.1. Synchronization Delay and Offset Metrics Reporting Blocks
This document defines two new block types to augment those defined in
[RFC3611], for use in a range of RTP applications.
The first new block type supports reporting of Initial
Synchronization Delay to establish multimedia session. Information
is recorded about time difference between the start of RTP sessions
and the time the RTP receiver acquires all components of RTP sessions
in the multimedia session [RFC6051].
The second new block type supports reporting of the relative
synchronization offset time of two arbitrary streams (e.g., between
audio and video streams), with the same RTCP CNAME included in RTCP
Source description items (SDES) packets [RFC3550].
These metrics belong to the class of transport level metrics defined
in [RFC6792].
1.2. RTCP and RTCP XR Reports
The use of RTCP for reporting is defined in [RFC3550]. [RFC3611]
defined an extensible structure for reporting using an RTCP Extended
Report (XR). This document defines a new Extended Report block for
use with [RFC3550] and [RFC3611].
1.3. Performance Metrics Framework
The RTP Monitoring Architectures [RFC6792] provides guideline for
reporting block format using RTCP XR. The new report block described
in this memo is in compliance with the monitoring architecture
specified in [RFC6792].
1.4. Applicability
When joining each session in layered video sessions [RFC6190] or the
multimedia session, a receiver may not synchronize playout across the
multimedia session or layered video session until RTCP Sender Report
(SR) packets have been received on all components of RTP sessions.
The component RTP session are referred to as each RTP session for
each media type in multimedia session or separate RTP session for
each layer in the layered video session. For multicast session, the
initial synchronization delay metric varies with the session
bandwidth, the number of members, and the number of senders in the
session. The RTP flow Initial synchronization delay block defined in
this document can be used to report such metric, i.e., the initial
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Internet-Draft SDO Report Blocks February 2014
synchronization delay to receive all the RTP streams belonging to the
same multimedia session or layered video session. In the absence of
packet loss, the initial synchronization delay equals to the average
time taken to receive the first RTCP packet in the RTP session with
the longest RTCP reporting interval. In the presence of packet loss,
the media synchronization should rely on the in-band mapping of RTP
and NTP-format timestamps [RFC6051] or wait until the reporting
interval has passed, and the next RTCP SR packet is sent.
Receivers of the RTP flow initial synchronization delay block could
use this metric to compare with targets (i.e., Service Level
Agreement or thresholds of the system) to help ensure the quality of
real-time application performance.
In an RTP multimedia session, there can be an arbitrary number of
streams carried in different RTP sessions, with the same RTCP CNAME.
These streams may be not synchronized with each other. For example,
one audio stream and one video stream belong to the same session, and
the audio stream is transmitted lagging behind video stream for
multiple tens of milliseconds [TR-126]. The RTP Flows
Synchronization Offset block can be used to report such
synchronization offset between video stream and audio stream. This
block is also applied to the case where an RTP session can contain
media streams with media from multiple media types. The metrics
defined in the RTP flows synchronization Offset block can be used by
the network manager for trouble shooting and dealing with user
experience issues.
2. Terminology2.1. Standards Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
In addition, the following terms are defined:
Initial Synchronization Delay:
A multimedia session comprises a set of concurrent RTP sessions
among a common group of participants, using one RTP session for
each media type. The initial synchronization Delay is the average
time for receiver to synchronize all components of a multimedia
session [RFC6051].
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Internet-Draft SDO Report Blocks February 2014
Synchronization Offset:
Synchronization between two media streams must be maintained to
ensure satisfactory Quality of Experience (QoE). Two media
streams can be of the same or different media type belonging to
one RTP session or in different media types belonging to one
multimedia session. The Synchronization Offset is the relative
time difference of the two media streams that need to be
synchronized.
3. RTP Flows Initial Synchronization Delay Report Block
This block is sent by RTP receivers and reports Initial
synchronization delay beyond the information carried in the standard
RTCP packet format. Information is recorded about time difference
between the start of multimedia session and the time when the RTP
receiver acquires all components of RTP sessions [RFC6051] measured
at the receiving end of RTP stream.
This block needs only be exchanged occasionally, for example sent
once at the start of RTP session.
3.1. Metric Block Structure
The RTP Flows Initial Synchronization Delay Report Block has the
following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT=RFISD | Reserved | Block length=2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of Source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initial Synchronization Delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Report Block Structure
3.2. Definition of Fields in RTP Flow Initial Synchronization Delay Metrics BlockAsaeda, et al. Expires August 30, 2014 [Page 5]

Internet-Draft SDO Report Blocks February 2014
Block type (BT): 8 bits
The RTP Flows Initial Synchronization Delay Report Block is
identified by the constant <RFISD>.
[Note to RFC Editor: please replace RFISD with the IANA provided
RTCP XR block type for this block.]
Reserved: 8 bits
This field is reserved for future definition. In the absence of
such a definition, the bits in this field MUST be set to zero and
ignored by the receiver.
Block length: 16 bits
The constant 2, in accordance with the definition of this field in
Section 3 of RFC 3611 [RFC3611].
SSRC of source: 32 bits
The SSRC of the media source SHALL be set to the value of the SSRC
identifier carried in any arbitrary component of RTP sessions
belonging to the same multimedia session.
Initial Synchronization Delay: 32 bits
The average delay, expressed in units of 1/65536 seconds, from the
beginning of multimedia session [RFC6051] to the time when RTCP
packets are received on all of the components RTP sessions. It is
recommended that the beginning of multimedia session is chosen as
the time when the receiver has joined the first RTP session of the
multimedia session. The value of the initial synchronization
delay is calculated based on received RTCP SR packets or the RTP
header extension containing in-band mapping of RTP and NTP-format
timestamps [RFC6051]. If there is no packet loss, the initial
synchronization delay is expected to be equal to the average time
taken to receive the first RTCP packet in the RTP session with the
longest RTCP reporting interval or the average time taken to
receive the first RTP header extension containing in-band mapping
of RTP and NTP-format timestamps.
If the measurement is unavailable, the value of this field with
all bits set to 1 MUST be reported.
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Internet-Draft SDO Report Blocks February 20144. RTP Flows Synchronization Offset Metrics Block
In the RTP multimedia sessions or one RTP session, there can be an
arbitrary number of Media streams and each media stream (e.g., audio
stream or video stream) is sent in a separate RTP stream. In case of
one RTP session, each media stream or each medium uses different
SSRC. The receiver associates RTP streams to be synchronized by
means of RTCP CNAME contained in the RTCP Source Description (SDES)
packets [RFC3550].
This block is sent by RTP receivers and reports synchronization
offset of two arbitrary RTP streams that needs to be synchronized in
the RTP multimedia session. Information is recorded about the
relative average time difference between two arbitrary RTP streams
(one is reporting stream, the other is reference stream) with the
same CNAME and measured at the receiving end of RTP stream. In order
to tell what the offset of reporting stream is relative to, the block
for reference stream with synchronization offset of zero should be
reported.
Instances of this Block refer by Synchronization source (SSRC) to the
separate auxiliary Measurement Information block [RFC6776] which
describes measurement periods in use (see [RFC6776] section 4.2).
This metrics block relies on the measurement period in the
Measurement Information block indicating the span of the report and
SHOULD be sent in the same compound RTCP packet as the measurement
information block. If the measurement period is not received in the
same compound RTCP packet as this Block, this Block MUST be
discarded.
4.1. Metric Block Structure
The RTP Flow General Synchronization Offset Report Block has the
following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT=RFSO | I | Reserved | Block length=3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Synchronization Offset, most significant word |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Synchronization Offset, least significant word |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Report Block Structure
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Internet-Draft SDO Report Blocks February 20144.2. Definition of Fields in RTP Flow General Synchronization Offset Metrics Block
Block type (BT): 8 bits
The RTP Flow General Synchronization Offset Report Block is
identified by the constant <RFSO>.
[Note to RFC Editor: please replace RFSO with the IANA provided
RTCP XR block type for this block.]
Interval Metric Flag (I): 2 bits
This field is used to indicate whether the Burst/Gap Discard
Summary Statistics metrics are Sampled, Interval or Cumulative
metrics:
I=10: Interval Duration - the reported value applies to the
most recent measurement interval duration between successive
metrics reports.
I=11: Cumulative Duration - the reported value applies to the
accumulation period characteristic of cumulative measurements.
I=01: Sampled Value - the reported value is a sampled
instantaneous value.
In this document, the value I=00 is the reserved value and MUST
NOT be used. If the value I=00 is received, then the XR block
MUST be ignored by the receiver.
Reserved: 6 bits
This field is reserved for future definition. In the absence of
such a definition, the bits in this field MUST be set to zero and
MUST be ignored by the receiver.
Block length: 16 bits
The constant 3, in accordance with the definition of this field in
Section 3 of RFC 3611 [RFC3611].
SSRC of Source: 32 bits
The SSRC of the media source SHALL be set to the value of the SSRC
identifier of the reporting RTP stream to which the XR relates.
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Internet-Draft SDO Report Blocks February 2014
Synchronization Offset: 64 bits
The synchronization offset of the reporting RTP stream relative to
the reference stream with the same CNAME. The calculation of
Synchronization Offset is similar to Difference D calculation in
the RFC3550. That is to say, if Si is the NTP timestamp from the
reporting RTP packet i, and Ri is the time of arrival in NTP
timestamp units for reporting RTP packet i, Sj is the NTP
timestamp from the reference RTP packet j, and Rj is the time of
arrival in NTP timestamp units for reference RTP packet j, then
the value of the synchronization offset D may be expressed as
D(i,j) = (Rj - Ri) - (Sj - Si) = (Rj - Sj) - (Ri - Si)
If in-band delivery of NTP-format timestamps is supported
[RFC6051], Si and Sj should be obtained directly from the RTP
packets where NTP timestamps are available. If not, Si and Sj
should be calculated from their corresponding RTP timestamps. The
value of the synchronization offset is represented using a 64-bit
signed NTP-format timestamp as defined in [RFC5905], which is 64-
bit signed fixed-point number with the integer part in the first
32 bits and the fractional part in the last 32 bits. A positive
value of the synchronization offset means that the reporting
stream leads before the reference stream, while a negative one
means the reporting stream lags behind the reference stream. The
synchronization offset of zero means the stream is the reference
stream.
If the measurement is unavailable, the value of this field with
all bits set to 1 MUST be reported.
5. SDP Signaling
[RFC3611] defines the use of SDP (Session Description Protocol)
[RFC4566] for signaling the use of XR blocks. XR blocks MAY be used
without prior signaling.
5.1. SDP rtcp-xr-attrib Attribute Extension
Two new parameters are defined for the two report blocks defined in
this document to be used with Session Description Protocol (SDP)
[RFC4566] using the Augmented Backus-Naur Form (ABNF) [RFC5234].
They have the following syntax within the "rtcp-xr" attribute
[RFC3611]:
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